1. Technical Field of the Invention
The present invention relates generally to rotors for rotating electrical machines, such as electric generators and motors, and methods of manufacturing the rotors. More particularly, the invention relates a rotor for an automotive alternator, which includes a pair of lundell-type pole cores, and its manufacturing methods.
2. Description of the Related Art
In a conventional automotive alternator, a rotor generally includes a pair of lundell-type pole cores which have the same shape and are opposed on a rotary shaft so as to abut each other. (For example, Japanese Patent First Publication H11-220845 discloses such a rotor.)
More specifically, each of the pole cores includes a cylindrical base portion fitted on the rotary shaft, a plurality of prismatic claw-base portions that each extend radially outward from an axially outer part of the base portion and are arranged in the circumferential direction of the rotary shaft at predetermined intervals, and a plurality of claw portions each of which axially extends from one of the claw-base portions. Further, the claw portions of one of the pole cores are interleaved with those of the other pole core, surrounding a field coil that is wound around the base portions of the pole cores. In addition, the rotor is so arranged in the automotive alternator that the claw portions of the pole cores are surrounded by a stator core.
In order to secure the mechanical strength of such a conventional rotor even when it rotates at high speeds, U.S. Pat. No. 4,377,762 discloses a method of fastening the pole cores to the rotary shaft.
More specifically, according to the disclosed method, the rotary shaft has a plurality of axially-extending protuberances on the outer surface thereof and annular grooves formed at axial positions corresponding to axially outer end faces of the pole cores. Each of the pole cores has a through-hole that is formed through the center of the base portion of the pole core. In assembly of the rotor, the rotary shaft is first pressed into the through-holes of the pole cores with the annular grooves being positioned slightly axially inward of the corresponding end faces of the pole cores. Then, staking crimp is applied to the pole cores to securely fasten them onto the rotary shaft.
Moreover, the pole cores of such a conventional rotor are generally forged using soft steel. However, due to limited precision of the forging equipment, there will be variations in parallelism between the end faces of the pole cores.
More specifically, referring to FIG. 11, the through-hole 178 of a pole core 107 can be formed based on either of several different reference faces. For example, the through-hole 178 can be formed to be perpendicular to either the axially outer end face 172 or the axially inner end face 176 of the pole core 107. Alternatively, the through-hole 178 can also be formed to be perpendicular to the bottom face 177 of an annular recess formed between the base portion and the claw portions of the pole core 107.
FIGS. 12A-15B illustrate a conventional rotor assembly process, wherein the through-hole 178 of each of the pole cores 107a and 107b is formed to be perpendicular to the axially outer end face 172.
Referring first to FIGS. 12A-12B, a pressing machine 130 is used for pressing the rotary shaft 105 into the through-holes 178 of the pole cores 107a and 107b, which includes a punch portion 131 and a die portion 132.
The punch portion 131 includes a punch 140 that is configured to move upward and downward. Further, the punch 140 includes therein a shaft holder 139 for holding the shaft 105 to be parallel to the vertical direction.
The die portion 132 includes a base 133, a bearing 134 provided on the base 133, and a mount 135 that is supported by the base 133 through the bearing 134. The mount 135 has center a through-hole 378 within which a center holder 136 is slidably disposed. The bearing 134 also serves to move the mount 135 horizontally, when the center of the mount 135 does not coincide with that of the center holder 136, thereby bringing the centers of the mount 135 and the center holder 136 into coincident with each other. Further, in the die portion 132, there are provided at least two claw-like holders 137 fixed to the base 133. The claw-like holders 137 work to press both axially and radially the pole core 107b, which is set on the mount 135 as a work, so as to hold it on the mount 135. In addition, it is possible to selectively make, using a lever 138, the claw-like holders 137 either hold or release the work set on the mount 135 (i.e., the pole core 107b in this case).
The conventional rotor assembly process includes the following steps.
At the first step, the pole core 107b is set on the mount 135, as shown in FIG. 12A. Since the pole core 107b is formed with the through-hole 178 being perpendicular to the axially outer end face 172, after the setting, the through-hole 178 and the axially outer end face 172 of the pole core 107b are respectively parallel to the vertical and horizontal directions with respect to the mount 135. Then, the center holder 136 is moved upward to be inserted into the through-hole 178 of the pole core 107b. With this insertion, the centers of the punch 140 and the mount 135 are definitely brought into alignment with each other. Thereafter, the claw-like holders 137 are made, by an operation of the lever 138, to press the pole core 107b, thereby holding the pole core 107b on the mount 135.
At the second step, as shown in FIG. 12B, the punch 140 is moved downward with an upper end portion of the rotary shaft 105 being vertically held by the shaft holder 139 and a lower end of the rotary shaft 105 abutting an upper end of the center holder 136. Consequently, the rotary shaft 105 is smoothly pressed into the through-hole 178 of the pole core 107b while being kept in parallel with the vertical direction.
When the rotary shaft 105 is further moved downward through the through-hole 178 of the pole core 107b to a predetermined position, the shaft holder 139 releases the rotary shaft 105, and the punch 140 is moved upward to return to an initial rest position thereof.
After that, the claw-like holders 137 are made by another operation of the lever 138 to release the pole core 107b; then, the pole core 107b is dismounted from the mount 135 with the rotary shaft 105 press-fit in the through-hole 178.
At the third step, referring to further FIG. 13A, the pole core 107a, which has the field coil 108 mounted thereto and is placed on a pallet, is griped and taken off from the pallet by a gripper 160 of the punch portion 131. Then, gripping the pole core 107a, the gripper 160 is moved downward, thereby setting the pole core 107a along with the field coil 108 on the mount 135 of the die portion 132. Thereafter, the gripper 160 releases the pole core 107a and is moved upward to an initial rest position thereof. At the same time, the center holder 136 is moved upward to be inserted into the through-hole 178 of the pole core 107a; then, the claw-like holders 137 are made by an operation of the lever 138 to press the pole core 107a, thereby holding the pole core 107a on the mount 135.
At the fourth step, referring to FIG. 13B, the gripper 160 grips the pole core 107b, which has the rotary shaft 105 press-fit therein, in such a manner that the rotary shaft 105 is parallel to the vertical direction. Then, with the lower end of the rotary shaft 105 abutting the upper end of the center holder 136, the gripper 160 is moved downward to press the rotary shaft 105 into the through-hole 178 of the pole core 107a. 
When the rotary shaft 105 is further moved downward through the through-hole 178 of the pole core 107a to a predetermined position, the gripper 160 releases the rotary shaft 105 and is moved upward to return to the initial rest position thereof.
After that, the claw-like holders 137 are made by another operation of the lever 138 to release the pole core 107a; then, the pole cores 107a and 107b are dismounted from the mount 135 with the field coil 108 sandwiched therebetween and the rotary shaft 105 press-fit in the through-holes 178.
In addition, it is also possible to first assemble together the pole cores 107a and 107b with the field coil 108 sandwiched therebetween and then press the rotary shaft 105 into the through-holes 178 of the pole cores 107a and 107b. 
At the fifth step, referring to FIG. 14, staking crimp is applied, using a crimp machine 150, to the pole cores 107a and 107b to securely fix them onto the rotary shaft 105.
Staking crimp is a process through which annular grooves are to be formed on the axially outer end faces 172 of the pole cores 107a and 107b around the rotary shaft 105, thereby forming plastic flows from the pole cores 107a and 107b to fill corresponding annular grooves provided on the rotary shaft 105.
The crimp machine 150 has a configuration similar to that of the pressing machine 130 described above. However, unlike the pressing machine 130, the punch portion 151 and the die portion 152 of the crimp machine 150 include a crimp punch 153 and a crimp die 154, respectively.
At this step, the pole cores 107a and 107b having the rotary shaft 105 press-fit therein and the field coil 108 mounted thereto are first set on the mount 155.
The mount 155 has the crimp die 154 concentrically disposed therein; further, the crimp die 154 has the center holder 156 concentrically and slidably disposed therein. The axis of the rotary shaft 105 is aligned with both the axes of the crimp punch 153 and the crimp die 154; thus, the axially outer end faces 172 of the pole cores 107b and 107a are respectively perpendicular to the axes of the crimp punch 153 and the crimp die 154.
With the above configuration, staking crimp is performed on the pole cores 107a and 107b when the crimp punch 153 is moved downward to press those against the crimp die 154.
As a result, through the above-described five steps, the rotor 103 is finally obtained.
The conventional rotor manufacturing method as described above may involve, however, the following problems.
When the through-holes 178 of the pole cores 107a and 107b are formed to be perpendicular to the axially outer end faces 172, the axially inner end faces 176 may not be perpendicular to the through-holes 178.
Consequently, after assembly of the rotor 103, the axially inner end faces 176 of the pole cores 107a and 107b may not be parallel to each other and thus may only partially contact with each other, resulting in an increased axial length of the rotor 103. Further, when staking crimp is applied to the pole cores 107a and 107b with the axially inner end faces 176 only in partial contact, it is difficult to secure high strength of the staking crimp. As a result, during rotation of the rotor 3, the pole cores 107a and 107b may come off the rotary shaft 105.
Otherwise, as illustrated in FIG. 15A, when the axially inner end faces 176 are brought into complete contact with each other by applying an excessive pressing force on the pole cores 107a and 107b, a bending moment will act on the rotary shaft 105. Further, when the bending moment exceeds an allowable limit for elastic deformation, the rotary shaft 105 will be inelastically deformed after the pressing step, as illustrated in FIG. 15B. Consequently, during operation, run out of the rotor 103 will be large, thus causing interference between the pole cores 107a and 107b and the stator core and increasing vibrations and rotational variation of the rotor 103.
In addition, to reduce run out of the rotor 103, finish machining may be applied to the pole cores 107a and 107b to improve the parallelism between the axially inner end faces 176 thereof. However, this will increase the steps of manufacturing and thus the cost of the rotor 103.